1. Field of the Invention
This invention generally relates to a method and apparatus for receiving and monitoring various signals (e.g. seismic, pressure, and temperature signals) in a borehole and more particularly to a process for installing an array of sensors inside a well in order to carry out extremely diverse measurements concerning the state of the well, to monitor flows inside the well, and to determine the evolution of the reservoir over time.
2. Description of the Related Art
During the production of hydrocarbons from an underground reservoir or formation, it is important to determine the development and behavior of the reservoir and to foresee changes which will affect the reservoir. Methods and apparatus for determining and measuring downhole parameters for forecasting the behavior of the reservoir are well known in the art.
A typical method and apparatus includes placing one or more sensors downhole adjacent the reservoir and recording seismic signals generated from a source often located at the surface. Hydrophones, geophones, and accelerometers are three typical types of sensors used for recording such seismic signals. Hydrophones respond to pressure changes in a fluid excited by seismic waves, and consequently must be in contact with the fluid to fimction. Hydrophones are non-directional and respond only to the compressional component of the seismic wave. They can be used to indirectly measure the shear wave component when the shear component is converted to a compressional wave (e.g. at formation interfaces or at the wellbore-formation interface). Geophones measure both compressional and shear waves directly They include particle velocity detectors and typically provide three-component velocity measurement. Accelerometers also directly measure both compressional and shear waves directly, but instead of detecting particle velocities, accelerometers detect accelerations, and hence have higher sensitivities at higher frequencies. Accelerometers are available having three-component acceleration measurements. Both geophones and accelerometers can be used to determine the direction of arrival of the incident elastic wave. One method which has been used to accomplish well logging or vertical seismic profiling is attaching the sensor to a wireline sonde and then lowering the wireline sonde into the bore of the well. See for example UK Patent Application GB 2,229,001A and "Permanent Seismic Monitoring, A System for Microseismology Studies" by Createch Industrie France, both incorporated herein by reference. U.S. Pat. No. 5,607,015, incorporated herein by reference, discloses installing an array of sensors suspended on a wireline into the well.
Wireline sondes contain a large number of various sensors enabling various parameters to be measured, especially acoustic noise, natural radioactivity, temperature, pressure, etc. The sensors may be positioned inside the production tubing for carrying out localized measurements of the nearby annulus or for monitoring fluid flowing through the production tubing.
In the case of geophones and accelerometers, the sensors must be mechanically coupled to the formation in order to make the desired measurement. UK Patent Application GB 2,307,077A, incorporated herein by reference, discloses providing the wireline sonde with an arm which can be extended against the wall of the casing. When extended, the arm presses ("clamps") the sensor against the opposite wall of the casing with a clamping force sufficient to prevent relative motion of the sensor with respect to the casing. As a rule of thumb, the clamping force should be at least five times the weight of the sensor, and it is not uncommon for sensors to weigh 30 lbs. or more.
Another method includes attaching sensors to the exterior of the casing as it is installed in the well. The annulus around the casing is then cemented such that when the cement sets, the sensors are permanently and mechanically coupled to the casing and formation by the cement. See for example U.S. Pat. Nos. 4,775,009 and 5,467,823 and EP 0 547 961 A1, all incorporated herein by reference.
One proposed use for sensor arrays includes the real-time monitoring of a fracture as it is being formed in a formation. These systems use arrays of acoustical energy sensors (e.g. geophones, hydrophones, etc.) which are located in a well that is in acoustical communication with the formation to detect the sequence of seismic events (e.g. shocks or "mini earthquakes") which occur as the formation is being hydraulically fractured. The sensors convert this acoustic energy to signals which are transmitted to the surface for processing to thereby develop the profile of the fracture as it is being formed in the formation. This monitoring is particularly useful when the hydraulic fracturing is performed for disposing waste materials in subterranean formations. Certain waste materials may be injected as a slurry into earth formations: e.g. see U.S. Pat. Nos. 4,942,929 and 5,387,737. The sensor arrays are then used to ensure the fracture (and hence the waste material) does not encroach into neighboring formations.
Well logging, whether from wireline or drill stem, only provides a very limited amount of information about the hydrocarbon reservoir. Monitoring and understanding formation subsidence and fluid movement in the interwell spacing is critical to improving the volume of hydrocarbons recovered from the reservoir and the efficiency with which they are recovered. One method for monitoring is time lapse seismic monitoring.
Subsidence of the strata within and above a reservoir may take place during hydrocarbon production because of movement and withdrawal of fluids. This subsidence and pore pressure changes caused by movement of fluids may cause tiny earthquakes. These "micro-earthquakes" may be detected by very sensitive seismic sensors placed in the wellbore near the microearthquake activity. Continuous seismic monitoring of such detected activity offers the possibility of monitoring subsidence and fluid migration patterns in reservoirs. Reservoirs are complicated and knowledge is needed to predict their flow paths and barriers.
Most of the cost of 3-D surveys is in data acquisition which is currently being done with temporary arrays of surface sources and receivers. Long-term emplacement of the receivers has the potential of lowering significantly data acquisition costs. There are two important reasons for long-term emplacement of receivers, first, repeatability is improved and second, by positioning the receivers closer to the reservoir, noise is reduced and vertical resolution of the seismic information is improved. Further, from an operational standpoint, it is preferred that receivers be placed in the field early to provide the capability of repeating 3-D surveys at time intervals more dependent on reservoir management requirements than on data acquisition constraints.
One method to determine the time evolution of a reservoir under production is the three dimensional vertical seismic profile (VSP). This method comprises the reception of waves returned by various underground reflectors by means of an array of geophones arranged at various depths inside the well, these waves having been transmitted by a seismic generator disposed on the surface or possibly inside another well. By obtaining a sequence of records distributed over a period of six months to many (e.g. ten) years, it becomes possible to monitor the movement of fluid in the reservoirs, and to thereby obtain information needed to improve the volume of recovered hydrocarbons and the efficiency with which they are recovered.
Long-term borehole sensor arrays for seismic monitoring must consist of many levels of sensors in order to provide sufficient reservoir coverage. Monitoring a reservoir with long-term seismic sensors requires many more sensors than those being used merely to monitor pressure and temperature in a wellbore. Pressure and temperature monitoring typically consists of a single sensor level near the producing zone.
Further, the general approach used for deploying arrays of downhole geophones has been to adapt surface seismic data acquisition cables to the downhole applications. Typically the downhole installations have used conventional geophones packaged in some hardened module with each geophone connected to the surface with a twisted pair of copper wires. Analog telemetry over twisted-pair copper wire has major disadvantages for large numbers of sensors. A large diameter umbilical cable is necessary because of the individual wires required for each sensor. Since molded connectors tend to be the main failure points, increasing the number of sensors also increases the number of connectors and increases the probability of failure in the sensor array. Further only low telemetry rates can be achieved. Seismic data for 3-D monitoring of reservoirs is vastly larger in quantity than for pressure and temperature monitoring. Further, storing any significant amount of data downhole is not practical. The data must be transmitted real time.
One deficiency of the prior art is protecting the umbilical cable from damage during emplacement. As arrays of sensors strapped to the outside of a string of pipe pass the bends and turns in the outer casing, they are subjected to shear and compression forces. These have caused many sensors and umbilical cables to be damaged or broken.
The present invention overcomes these deficiencies of the prior art.